This invention relates to the heat treatment of steel articles, and in particular relates to induction heating, quenching, and tempering of steel sheets.
In order to improve the mechanical properties of metal articles, metal is typically subjected to time consuming, and therefore costly, heat treatment processes. To increase the hardness of a steel, a steel article may be subjected to a heating cycle at or above a temperature of the metal's critical temperature, followed by quenching the metal article. This process typically results in creation of a martensitic microstructure in steels. Martensitic microstructures, while very hard, are also known to be relatively brittle, i.e., having little ductility. To increase the ductility of martensitic microstructures, such steels are often tempered, or heated to a temperature below the steel's critical temperature, whereby stresses built up in the steel during quenching are reduced. Such heating, quenching, and tempering processes are typically long to conduct, and accordingly, expensive.
In processing steel generally, and, more specifically, in forming anti-ballistic armor, it has until now been difficult to achieve a metal product having a combination of strength and ductility which could be manufactured without high cost, including extensive heat treatment time. For example, such a metal article should be able to resist penetration by armor piercing ammunition as well as fragments from improvised explosive devices, including explosively formed projectiles. We have found a method and apparatus for heat treating, quenching, and tempering a steel article whereby the article has desirable mechanical and microstructure properties, including properties which may be useful in acting as anti-ballistic armor or in other applications which may require a steel sheet having high hardness in combination with high ductility.
Disclosed is a method for treating a steel article to form a high hardness and high ductility alloy comprising the steps of:
(a) providing a steel composition having a material thickness less than 0.5 inches (12.7 mm), having an initial microstructure of ferrite and pearlite, and having a composition of, by weight,
carbon between 0.25 and 0.55%,
silicon between 0.15 and 0.35%,
manganese between 0.40 and 1.0%,
chromium between 0.80 and 1.10%,
molybdenum between 0.15 and 0.25%,
sulfur less than 0.040%,
phosphorus less than 0.035%;
(b) preheating the provided steel composition at least 0.7° C. (33.3° F.) per second to not more than 538° C. (1000° F.);
(c) heating the provided steel composition to a peak temperature of between 850° C. (1562° F.) and 1150° C. (2102° F.) in less than ten seconds;
(d) holding the heated steel composition at a temperature within the peak temperature range for between two and ten seconds;
(e) quenching the heated steel composition from the peak temperature range to below 100° C. (212° F.) at a temperature rate reduction of between 400 and 3000° C./sec (752-5432° F./sec);
(f) removing residual quench media from the surface of the quenched steel composition;
(g) tempering the quenched steel composition at a temperature from 100° C. to 260° C. (212-500° F.) for less than ninety minutes;
(h) air cooling the tempered steel composition to less than 100° C. (212° F.) to form a steel article having a transformed microstructure at least 80% martensite and up to 5% bainite, a yield strength of at least 1800 MPa, a total elongation between 5% and 12%, and having a V50 protection ballistic limit at 30° obliquity angle between 2200 and 2700 feet per second (670-823 m/s) with a .30 caliber armor piercing round for a thickness of 0.25″ (6.35 mm).
Alternatively, disclosed is a method for treating a steel article to form a high hardness and high ductility alloy comprising the steps of:
(a) providing a steel composition having a material thickness less than 0.5 inches (12.7 mm), having an initial microstructure of ferrite and pearlite, and having a composition of, by weight,
carbon between 0.25 and 0.55%.
silicon between 0.15 and 0.35%,
manganese between 0.40 and 1.0%,
chromium between 0.80 and 1.10%,
molybdenum between 0.15 and 0.25%,
sulfur less than 0.040%,
phosphorus less than 0.035%;
(b) preheating the provided steel composition at least 0.7° C. (33.3° F.) per second to between the austenization temperature and 815° C. (1500° F.);
(c) heating the provided steel composition to a peak temperature of between 850° C. (1562° F.) and 1150° C. (2102° F.) in less than ten seconds;
(d) holding the heated steel composition at a temperature within the peak temperature range for between two and sixty seconds;
(e) quenching the heated steel composition from the peak temperature range to below 100° C. (212° F.) at a temperature rate reduction of between 400 and 3000° C./sec (752-5432° F./sec);
(f) removing residual quench media from the surface of the quenched steel composition;
(g) tempering the quenched steel composition at a temperature from 100° C. to 260° C. (212-500° F.) for less than ninety minutes;
(h) air cooling the tempered steel composition to less than 100° C. (212° F.) to form a steel article having a transformed microstructure at least 80% martensite and up to 5% bainite, a yield strength of at least 1800 MPa, a total elongation between 5% and 12%, and having a V50 protection ballistic limit at 30° obliquity angle between 2200 and 2700 feet per second (670-823 m/s) with a .30 caliber armor piercing round for a thickness of 0.25″ (6.35 mm).
Also disclosed is a method for treating a steel article to form a high strength and high ductility alloy comprising the steps of:
(a) providing a steel composition having a material thickness less than 0.5 inches (12.7 mm), having an initial microstructure of ferrite and pearlite, and having a composition of, by weight,
carbon between 0.25 and 0.55%
silicon between 0.15 and 0.35%,
manganese between 0.40 and 1.0%,
chromium between 0.80 and 1.10%,
molybdenum between 0.15 and 0.25%,
sulfur less than 0.040%,
phosphorus less than 0.035%;
(b) preheating the provided steel composition to a temperature between 260° C. (500° F.) and 538° C. (1000° F.).
(c) heating the preheated steel composition to a peak temperature of between 850° C. (1562° F.) and 1150° C. (2102° F.) in less than ten seconds;
(d) holding the heated steel composition at the peak temperature range for between two and sixty seconds;
(e) quenching the heated steel composition from the peak temperature range to below 100° C. (212° F.) at a temperature rate reduction of between 400 and 3000° C./sec (752-5432° F./sec);
(f) removing residual quench media from the surface of the quenched steel composition;
(g) tempering the quenched steel composition at a temperature from 100° C. to 260° C. (212-500° F.) for less than ninety minutes;
(h) air cooling the tempered steel composition to less than 100° C. (212° F.) to form a steel article having at least 80% martensite and up to 5% bainite, a yield strength of at least 1800 MPa, a total elongation between 5% and 12%, and having a V50 protection ballistic limit at 30° obliquity angle between 2300 and 2510 feet per second (701-765 m/s) with a .30 caliber armor piercing round for a thickness of 0.25″ (6.35 mm).
Alternatively, disclosed is a method for treating a steel article to form a high strength and high ductility alloy comprising the steps of:
(a) providing a steel composition having a material thickness less than 0.5 inch (12.7 mm), having an initial microstructure of ferrite and pearlite, and having a composition of, by weight,
carbon between 0.25 and 0.55%
silicon between 0.15 and 0.35%,
manganese between 0.40 and 1.0%,
chromium between 0.80 and 1.10%,
molybdenum between 0.15 and 0.25%,
sulfur less than 0.040%,
phosphorus less than 0.035%;
(b) preheating the provided steel composition at least 0.7° C. (33.3° F.) per second to not more than 538° C. (1000° F.);
(c) heating the preheated steel composition to a peak temperature between 850-1150° C. (1562-2102° F.) in less than ten seconds;
(d) holding the heated steel composition at the peak temperature range for between two and sixty seconds;
(e) quenching the heated steel composition to below 100° C. (212° F.) in less than four seconds;
(f) removing residual quench media from the surface of the quenched steel composition;
(g) tempering the quenched steel composition at a temperature between 100° C. and 260° C. (212-500° F.) for less than ninety minutes;
(h) air cooling the tempered steel composition to less than 100° C. (212° F.) having a transformed microstructure of at least 80% martensite and up to 5% bainite, a yield strength of at least 1800 MPa, and a total elongation between 5% and 12%.
Additionally, prior to heating the steel composition, two or more lengths of steel plates may be welded together along the width with one or more welds to form a continuous series of steel plates. Further, the step of welding may include applying a weave weld bridging between lengths of steel plate across the width of the steel plates. Further, the step of welding may include applying a weave weld bridging between lengths of steel plate in three sections where the center portion of steel plate is welded first and the side portions are welded to provide a weave weld across the width of the steel plates. In any event, a seam weld is applied over the weave weld across the width of the steel plates. Further, an indicia may be applied to the steel plate in advance of the welding step to enable a vision system to identify the location of end portions of lengths of the steel plates for the welding step.
During the quenching step, the heated steel composition again may be quenched from the peak temperature range to below 100° C. (212° F.) at a temperature rate reduction of between 400 and 3000° C./second (752-5432° F./sec). In the disclosed method, the quenching step may be performed by flowing a quench medium over the steel article at a rate of up to 900 gallons/min (3400 L/min). In one alternative, the quench medium may be water. Following quenching, the residual quench media may be removed from the surface of the quenched steel composition by at least one of mechanical wiping, blown air, and combinations thereof.
The quenched steel composition may be induction tempered for less than ten minutes, while in one alternative the quenched steel composition may be oven tempered for less than ninety minutes, and in another alternative the quenched steel composition may be tempered by a combination of oven and induction tempering for 30-60 minutes. The quenching step may be, for example, performed in more than 1 second and not more than 20 seconds. In still yet another alternative, the quenched steel composition may be induction tempered for two minutes or less. The tempering step may be performed at between 120° C. (250° F.) and 240° C. (500° F.). After quenching or tempering step, the steel plate may be cut into lengths at least at the seams to make substantially rectangular processed steel product while the steel plate continuously moves along the conveyor.